Method of detecting changes of a specular surface

ABSTRACT

A method of detecting small dimensional surface changes in a specular object or of comparing two nearly identical specular objects comprises illuminating a holographic plate by a plane reference wave and by an object wave produced by specularly reflecting a plane illuminating wave from the object and then illuminating the developed halogram with the conjugate of the reference wave so that the real object image is formed on the object. The light specularly reflected from the object will be a plane wave except to the extent that changes in the object surface have taken place. The nearly planar reflected wave is compared to a plane wave to detect the surface changes. Preferably, the object is originally illuminated by a patterned plane wave so that the final wave reflected from the object during reconstruction can be compared to a negative transparency of the pattern whereby surface changes are readily apparent.

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Upatniehs et a1. July 24, 1973 METHOD OF DETECTING CHANGES OF A PrimaryExaminer-Ronald L. Wibert SPECULAR SURFACE Assistant Examiner-Paul K.Godwin [75] Inventors: Juris Upatuieks, Ann Arbor; Philip Attorney JeanCarpenter warren et V. Mohan, Birmingham, both of Mich- 57 ABSTRACT [73]Asslgnee: Siz s S :21 28 Corporation A method of detecting smalldimensional surface changes in a specular object or of comparing two[22] Filed: Nov. 1, 1971 nearly identical specular objects comprisesilluminating a holographic plate by a plane reference wave and by [21]Appl' 194734 an object wave produced by specularly reflecting a planeilluminating wave from the object and then illu- [52] U.S. Cl 356/212,356/168, 350/35 minating the developed halogram with the conjugate of[51] Int. Cl. ..G1lln 21/48 the reference wave so that the real objectimage is [58] Field of Search 350/35; 356/212, formed on the object. Thelight specularly reflected 356/168 from the object will be a plane waveexcept to the extent that changes in the object surface have taken [56]References Cited place. The nearly planar reflected wave is compared toUNITED STATES PAT NTS I a plane wave to detect the surface changes.Preferably,

3,619,065 11/1971 Agnew; 356/168 x the is originally illuminated by aPamamed Plane 3,668,405 6/1972 Brooks et a1 350/35 x Wave that finalWave reflected {mm the Object during reconstruction can be compared to anegative OTHER PUBLICATIONS transparency of the pattern whereby surfacechanges Mask Defect Detection by Holograms" by Laming et are dilapparent a1., IBM Tech. Disc. Bltn., V01. 14, No. 1, June 1971.

4 Claims, 2 Drawing Figures 37480 15 a owmeewzrz:

METHOD OF DETECTING CHANGES OF A SPECULAR SURFACE This invention relatesto a method of detecting changes in a specular surface or differences insimilar surfaces by holography.

Previously it has been difficult to detect small dimensional deviationsin surfaces due to wear or deformation, for example, particularly wherethe object is large or has a complex contour. This is overcome forspecular objectsby the present invention.

A typical hologram is a photographic record of the pattern ofinterference between a simple light wave (reference wave) and a morecomplicated wave modified by the interaction with an object (objectwave). It is well known that in reconstructing an image from thehologram that a conjugate reference wave (the same as the original wavebut traveling in the opposite direction) constructs a conjugate objectwave in a holographic system. Since the conjugate of a plane wave isanother plane wave traveling in the opposite direction, the use of sucha wave simplifies the system. A conjugate reference wave is commonlyused to produce a real image of the object.

In this method a plane or other simple wave is used to illuminate theobject during hologram construction.

During reconstruction with the conjugate of the original reference beam,light traveling from the hologram to form the real image is reflectedfrom the object giving another plane or simple wave traveling in theopposite direction. It will differ from the original illuminating waveonly in regions of deformation of the object. It can be compared withthe original wave interferometrically to indicate such regions ofdeformation. If the original wave illuminating the object has a patternsuperimposed on it, the deformation will give changes in the pattern.

An object of this invention then is to provide a method forholographically detecting changes in a specular surface by reducing acomplex wavefront to a simple wavefront which is readily analyzed todetect surface changes.

A further object of the invention is to provide a method forholographically detecting changes in large or complex specular surfaces.

This invention is carried out by constructing a hologram ,of a specularobject using simple, preferably plane, waves to illuinate the object andto serve as the reference wave. Where it'is later desired to inspect theobject for surface changes due to wear or deformation or to compare theoriginal with another nearly identical object, the object image isreconstructed by illuminating the hologram with the conjugate of thereference wave so that the real image is directed onto the object andreflected therefrom to produce a wave like the original illuminatingwave except for those regions where deformation has occurred in theobject. Finally, the reflected wave is analyzed to detect regions ofdeformation of the object. Preferably the object is originallyilluminated with a patterned plane wave so that the final reflected wavefrom the object will bear the same pattern except for readily identifiedabberations due to deviations of object surfaces.

The above and other advantages will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts andwherein:

' onto the screen 32 from the object 20' will be visually forreconstructing the hologram of FIG. 1 and for de-' tecting changes inthe object according to the inven- 7 tion.

In FIG. 1 the optical system for constructing a hologram comprises alaser 10 which projects a beam of light to a beam splitter 12. A portionof that beam passes through the beam splitter to an expanding lens 14which projects a diverging beam onto a collimating lens 16. Thecollimated beam then passes through a patterned mask 18 and onto thespecular surface of an object 20 which reflects the beam onto aphotographic plate 22. Another portion of the laser beam is reflectedfrom the beam splitter 12 to mirror 24 and through an expanding lens 26and a collimating lens 28 to provide a plane reference wave projectedonto the plate 22. The mask is of any pattern in which distortionsthereof are readily detected by observation. The mask may be, forexample, a window screen or a transparency of a grid pattern.

FIG. 2 illustrates the reconstruction of an image from the hologram 22'which was formed by the development of the exposed photographic plate22. A laser 10' projects a beam through an expanding lens 14' and acollimating lens 16' onto the hologram 22 in such a direction that theplane waves reaching the hologram 22 .are the conjugate of the planereference beam usedduring hologram construction. The object 20' whichhas experienced some wear, deformation or other surface change since thetime of hologram construction or is another object nearly identicalto'the original object 20, is placed so that the real image of theobject registers on the object and is reflected therefrom through a mask30 and onto a viewing screen 32. The mask 30 is a negative transparencyof the mask 18 used during However, to the extent the object 20' differsfrom the object 20, the pattern reflected onto the mask 30 will bedeformed in those regions where the surface of the object 20 is deformedso that some light will pass the transparent portions of the mask 30 andwill be projected onto the screen 32 such that the regions of thesurface deformation of the object 20' will be readily apparent byviewing the screen. Alternatively, the mask 30 may be omitted so thatthe entire pattern reflected examined to determine what changes, if any,have taken place in the surface of the object. In a second alternateform of the invention, both mask 18 and 30 are omitted so that the beamreflected from the object 20' is free of any superimposed pattern. Inthat case, the reflected beam will be a plane wave if the objects 20 and20 are identical and will deviate from a plane wave to an extentcorresponding to the regions of the surface change. This deviation isthen detected by interfere metrically comparing the reflected wave witha plane wave.

It is not essential to use plane waves for construction orreconstruction of the hologram since other simple waves will suffice.Generally, however, plane waves will be most convenient for executionand analysis of surface deviations.

It will be seen that the object surface must be sufficiently specular toeffect sufficient reflection of the object image for analysis-Thus themethod is not suitable for detecting dimensional changes in highlydiffuse surfaces but on the other hand, it is not required that thesurface be perfectly specular. Accordingly, the term specular as usedherein means capable of reflecting a beam adequate for analysis.

A particular advantage of the method of this invention is thatinspection of large surfaces as well as complex surfaces is easilycarried out. This method also has the advantage of reducing a complexwavefront such as the original object wave or a reconstructed objectwave to a simple wavefront so that deviations are readily apparent.

The embodiment of the invention described herein is for purposes ofillustration and the scope of the invention is intended to be limitedonly by the following claims.

it is claimed:

1. The method of comparing one specular surface with another nearlyidentical specular surface comprising the steps of illuminating onespecular surface with a simple wave of coherent light,

constructing a hologram by illuminating a holographic plate with asimple reference wave of coherent light and with light specularlyreflected from the surface.

developing the hologram,

reconstructing the surface image by illuminating the hologram with theconjugate of the reference wave so that the real surface image isdirected onto the other surface and reflected therefrom to produce awave identical to the original surface illuminating wave to the extentthe two surfaces are identical,

and analyzing the reflected wave to determine its deviation from theoriginal illuminating wave to therefore detect differences in thesurface.

2. The method of detecting surface deformation of a specular objectcomprising the steps of illuminating a specular object prior todeformation with a plane wave of coherent light,

constructing a hologram by illuminating a holo- I graphic plate with aplane reference wave of coherent light and with light specularlyreflected from the object,

developing the hologram,

after object deformation reconstructing the object image by illuminatingthe hologram with the conjugate of the reference wave so that the realobject image is directed, onto the deformed object and reflectedtherefrom to produce a wave which is planar to the extent the objectsurface has not 3. The method of detecting surface deformation of a 5specular object comprising the steps of superimposing a pattern on aplane wave of coherent light,

illuminating a specular object prior to deformation with the plane wavebearing the superimposed pattern,

constructing a hologram by illuminating a holographic plate with a planereference wave of coherent light and with light specularly reflectedfrom the object,

developing the hologram,

after object deformation reconstructing the object image by illuminatingthe hologram with the conjugate of the reference wave so that the realobject image is directed onto the object and reflected therefrom toproduce a resultant patterned wave identical to the original patternedwave to the extent the object surface has not changed since the time ofhologram construction,

and comparing the pattern carried by the resultant patterned wave withthe original pattern to detec surface changes in the object.

4. The method of detecting surface deformation of a specular objectcomprising the steps of J superimposing a pattern on a plane wave ofcoherent light,

illuminating a specular object prior to deformation with the plane wavebearing the superimposed pattern,

constructing a hologram by illuminating a holographic plate with a planereference wave of coherent light and with light specularly reflectedfrom the object,

developing the hologram,

after object deformation reconstructingthe object image by illuminatingthe hologram with the conjugate of the reference wave-so that the realobject image is directed onto the object and reflected therefrom toproduce a resultant patterned wave identical to the original patternedwave to the extent the object surface has not changed since the time ofhologram construction,

and comparing the pattern carried by the resultant patterned wave withthe original pattern to detect surface changes in the object byprojecting the resultant patterned wave onto a mask bearing a patternwhich is the negative of the superimposed pattern so that only portionsof the patterned wave corresponding to surface changes pass through the1 mask.

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1. The method of comparing one specular surface with another nearlyidentical specular surface comprising the steps of illuminating onespecular surface with a simple wave of coherent light, constructing ahologram by illuminating a holographic plate with a simple referencewave of coherent light and with light specularly reflected from thesurface. developing the hologram, reconstructing the surface image byilluminating the hologram with the conjugate of the reference wave sothat the real surface image is directed onto the other surface andreflected therefrom to produce a wave identical to the original surfaceilluminating wave to the extent the two surfaces are identical, andanalyzing the reflected wave to determine its deviation from theoriginal illuminating wave to therefore detect differences in thesurface.
 2. The method of detecting surface deformation of a specularobject comprising the steps of illuminating a specular object prior todeformation with a plane wave of coherent light, constructing a hologramby illuminating a holographic plate with a plane reference wave ofcoherent light and with light specularly reflected from the object,developing the hologram, after object deformation reconstructing theobject image by illuminating the hologram with the conjugate of thereference wave so that the real object image is directed onto thedeformed object and reflected therefrom to produce a wave which isplanar to the extent the object surface has not changed since the timeof hologram construction, and analyzing the reflected wave to determineits in the object.
 3. The method of detecting surface deformation of aspecular object comprising the steps of superimposing a pattern on aplane wave of coherent light, illuminating a specular object prior todeformation with the plane wave bearing the superimposed pattern,constructing a hologram by illuminating a holographic plate with a planereference wave of coherent light and with light specularly reflectedfrom the object, developing the hologram, after object deformationreconstructing the object image by illuminating the hologram with theconjugate of the reference wave so that the real object image isdirected onto the object and reflected therefrom to produce a resultantpatterned wave identical to the original patterned wave to the extentthe object surface has not changed since the time of hologramconstruction, and comparing the pattern carried by the resultantpatterned wave with the original pattern to detect surface changes inthe object.
 4. The method of detecting surface deformation of a specularobject comprising the steps of superimposing a pattern on a plane waveof coherent light, illuminating a specular object prior to deformationwith the plane wave bearing the superimposed pattern, constructing ahologram by illuminating a holographic plate with a plane reference waveof coherent light and with light specularly reflected from the object,developing the hologram, after object deformation reconstructing theobject image by illuminating the hologram with the conjugate of thereference wave so that the real object image is directed onto the objectand reflected therefrom to produce a resultant patterned wave identicalto the original patterned wave to the extent the object surface has notchanged since the time of hologram construction, and comparing thepattern carried by the resultant patterned wave with the originalpattern to detect surface changes in the object by projecting theresultant patterned wave onto a mask bearing a pattern which is thenegative of the superimposed pattern so that only portions of thepatterned wave corresponding to surface changes pass through the mask.